Astronomers using the Atacama Large Millimeter/submillimeter Array (ALMA) and the Northern Extended Millimeter Array (NOEMA) have made the first definitive detection of a radioactive molecule in interstellar space. The observations reveal that this molecule — a form, or isotopologue of aluminum monofluoride (26AlF) — was dispersed into space after the collision of two stars, a rare cosmic event that was witnessed on Earth as a nova in 1670.
An artist’s impression of the collision of two stars, like the ones that formed CK Vulpeculae. The inset illustrates the inner structure of one red giant before the merger. A thin layer of 26Al (brown) surrounds a helium core. An extended convective envelope (not to scale), which forms the outermost layer of the star, can mix material from inside the star to the surface, but it never reaches deep enough to dredge 26Al up to the surface. Only a collision with another star can disperse 26Al. Image credit: NRAO / AUI / NSF / S. Dagnello.
When two Sun-like stars collide, the result can be a spectacular explosion and the formation of an entirely new star. One such event was seen from Earth in 1670. It appeared to observers as a bright, red nova, now known as CK Vulpeculae.
Though initially visible with the naked eye, this burst of cosmic light quickly faded and now requires powerful telescopes to see the remains of this merger: a dim central star surrounded by a halo of glowing material flowing away from it.
Over three centuries after this event, Harvard-Smithsonian Center for Astrophysics astronomer Tomasz Kamiński and co-authors studied the remains of this explosive stellar merger and discovered the clear and convincing signature of the radioactive isotope of aluminum (26Al) — which has a mean lifetime of 1.04 million years — bound with atoms of fluorine, forming 26AlF.
This is the first molecule bearing an unstable radioisotope definitively detected outside of our Solar System.
“The first solid detection of this kind of radioactive molecule is an important milestone in our exploration of the cool molecular Universe,” Dr. Kamiński said.
The team detected the unique spectral signature of these molecules in the debris surrounding CK Vulpeculae, which is 2,000 light-years from Earth.
As these molecules spin and tumble through space, they emit a distinctive fingerprint of millimeter-wavelength light, a process known as ‘rotational transition. Astronomers consider this the’ gold standard’ for molecular detections.
The observation of 26Al provides fresh insights into the merger process that created CK Vulpeculae.
It also demonstrates that the deep, dense, inner layers of a star, where heavy elements and radioactive isotopes are forged, can be churned up and cast into space by stellar collisions.
“We are observing the guts of a star torn apart three centuries ago by a collision,” Dr. Kamiński said.
This image shows CK Vulpeculae, the remains of a double-star collision. This impact launched radioactive molecules into space, as indicated in this artist’s impression, which gives a very close-up view of what these molecules look like. The background image was created from a combination of visible-light images from the Gemini telescope (blue), a submillimeter map showing the dust from the SMA (yellow) and finally a map of the molecular emission from APEX and the SMA (red). Image credit: L. Calçada / ESO.
The astronomers also determined that the two stars that merged were of relatively low mass, one being a red giant star with a mass somewhere between 0.8 and 2.5 times that of our Sun.
Being radioactive, 26Al will decay to become more stable and in this process one of the protons in the nucleus decays into a neutron. During this process, the excited nucleus emits a photon with very high energy, which we observe as a gamma ray.
Previously, detections of gamma ray emission have shown that around two solar masses of 26Al are present across our Milky Way Galaxy, but the process that created the radioactive atoms was unknown.
Furthermore, owing to the way that gamma rays are detected, their precise origin was also largely unknown.
At the same time, however, the scientists have concluded that the production of 26Al by objects similar to CK Vulpeculae is unlikely to be the major source of 26Al in the Milky Way.
The mass of 26Al in CK Vulpeculae is roughly a quarter of the mass of Pluto, and given that these events are so rare, it is highly unlikely that they are the sole producers of the isotope in the Milky Way. This leaves the door open for further studies into these radioactive molecules.
The research is published in the journal Nature Astronomy (arXiv.org preprint).
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Tomasz Kamiński et al. Astronomical detection of radioactive molecule 26AlF in the remnant of an ancient explosion. Nature Astronomy, published online July 30, 2018; doi: 10.1038/s41550-018-0541-x
